Re: It'll be intersting to see how Apple handles this.
It's actually a frustratingly easy mistake to make with Apple's APIs — those CFIndexes pop up in quite a few places — and Xcode ships with the implicit signedness conversions warning disabled. It's one of the things I always enable when I'm starting a new project. Just enabling that would probably help them catch stuff like this.
That said, if it's a latent problem in initial table setup then the true diagnosis is probably that whomever guarantees the signed value would always be positive needs fixing, so they'd probably just have thrown the explicit cast in and forgotten about it.